Acetic acid is commercially produced from methanol and carbon monoxide by methanol carbonylation in the presence of water. The process may further contain methyl acetate as a co-feed. As an alternative to methyl acetate, the reaction can take place in the presence of a mixture of methyl acetate and methanol from byproduct streams of the hydrolysis/methanolysis of polyvinyl acetate. Various techniques can be used to determine the components of the reaction mixture and to modify the process.